Little is known about the jellyfish - marine biologists do not even know how powerful its sting is. Nor are they sure whether the species’ sudden re-emergence in the Adriatic is linked to the effects of global warming.

Experts say that jellyfish such as Drymonema dalmatinum have two distinct phases in their lives - an early phase when they are bottom-dwelling polyps, and a secondary phase in which they coalesce into floating jellyfish.

It may be that this particular species spends decades living at the bottom of the sea before evolving into a fully-formed jellyfish and that its reappearance has nothing to do with warmer seas.

“The polips are normally small and can live for a long time,” said Ferdinando Boero, from Salento University in Puglia, one of Italy’s foremost experts on jellyfish.

“Every now and then they produce jellyfish. Some species remain small, others become much bigger,” he told La Stampa newspaper.